a) Field of the Invention
The invention is directed to a condenser arrangement for brightfield illumination and/or darkfield illumination for optical microscopes with a device for switching from brightfield illumination to darkfield illumination and cardioid condenser-type darkfield illumination optics suitable for this purpose.
b) Description of the Related Art
For the microscope user, it is important to view object details with different contrasts which can be realized by different types of illumination, e.g., brightfield illumination and/or darkfield illumination of the objects, in order to obtain more information from the object from different viewing angles. For example, alternating use of transmitted brightfield illumination and transmitted darkfield illumination of one and the same object detail is extremely informative. For example, in the clinical or biological laboratory, as well as in other areas, applications frequently aim at the identification and detection of structures that may be below the resolution limit of the microscope. In certain cases, it is impossible to detect a structure or to display it adequately when using only one contrast in brightfield.
In darkfield illumination in optical microscopy, the object to be observed is illuminated laterally so that the light scattered at the object can be observed through the optics of the microscope and evaluated. Using darkfield illumination, determined object structures can be brought out clearly and very small objects can be detected.
Alternate switching of an alternative contrast, preferably darkfield, is often useful. In practice, a satisfactory solution is reached when both types of illumination achieve the following performance features:    Brightfield: High aperture for good resolution and a large object field for detecting the largest possible section of the object.    Darkfield: Very high illumination aperture for detection of submicroscopic structures and the illumination of large object fields in darkfield for detecting the largest possible section of the object.
Methods and arrangements for the use of brightfield illumination and darkfield illumination individually and in combination with different condensers are known.
For example, DE 37 00 965 A1 discloses an illumination device which uses condensers with swivelable front optics for brightfield illumination with switching between a high aperture and small object field on one hand and a small aperture and large object field on the other hand. The darkfield illumination is carried out through a central annular diaphragm in the aperture diaphragm plane and in the condenser pupil, respectively. This solution functions only for a limited magnification range of the objectives.
Zeiss and Leica also offer devices that permit alternate switching between brightfield and darkfield in a combination condenser.
However, all of these arrangements have disadvantages with respective to alternative use. With special condensers, only brightfield is possible in the entire magnification range under optimum field conditions and aperture conditions, or only a rotationally symmetric darkfield in the medium magnification range or one-sided darkfield for low magnifications can be realized satisfactorily. While the combination condenser does deliver a good brightfield, the quality of the darkfield is limited. Another disadvantage of many combination condensers in which the aperture iris and turret plate are arranged with additional contrast elements at different heights in the beam path (condenser U-UCD 8, Olympus, Japan) consists in that the aperture diaphragm must be readjusted when switching to brightfield. It is necessary to adjust to the original diameter of the diaphragm again in order to realize the identical contrast conditions. An absolutely identical image impression can be achieved only with difficulty because the adjustment is judged subjectively. When changing from brightfield to other contrasts, e.g., darkfield, the aperture diaphragm must be opened completely in order to open the illumination channel at the annular diaphragm. This is also true for brightfield at low magnifications when the front optics of the condenser are eliminated.
Various optical arrangements are known for realizing a darkfield illumination of microscopic objects with or without the intermediary of an immersion liquid.
For example, illumination of this kind can be realized through the use of suitable annular diaphragms in the entrance-side condenser pupil.
To achieve this same goal, JP 10268205 discloses plane mirror arrangements which deflect the light into the object plane by means of stepped reflectors or light ladders with a high aperture.
Further, there are known cardioid systems with curved reflecting surfaces enabling aplanatic illumination of the object plane with a high aperture.
JP 11153755 discloses the use of a ring of toric micromirrors that deflects the light to the object plane at a high aperture.
An arrangement of concave and convex annular mirrors for darkfield illumination in microscopes is known from DR 830 840 and DE 24 10 874. All of these arrangements have the disadvantage that only relatively small object fields can be illuminated and, therefore, they are suitable only for high magnifications.
In order to illuminate larger object fields as well, DE 34 25 674 proposes providing a toroidal annular mirror. It is disadvantageous that the illumination is inhomogeneous for larger object fields and an additional diffusion plate must be used for homogenization. However, this reduces the light yield in the object plane.
Further, DR 608 644 describes a condenser in which pyramid-shaped partial mirror surfaces which face one another are used as main mirrors and complementary mirrors. Because of the principle employed, this arrangement also has inhomogeneities in the illumination. Further, it is difficult to realize the concave mirror as a segmented mirror with the required accuracy.
For low magnifications, there are also known solutions which use a stepped mirror with three mirror surfaces, one of which has stamped, toric micromirrors.
In the past, different illumination optics have always been used to achieve a darkfield illumination, with or without the use of immersion, because it is very difficult at high numerical apertures and high magnifications to realize optical arrangements which are suitable for both dry illumination and immersion illumination in darkfield.